This invention relates generally to an apparatus for controlling the distribution of evaporated material onto a surface and particularly to such an apparatus for aluminumizing the inside surface of kinescope faceplates.
An image is produced on the inside surface of a cathode-ray tube (CRT), or picture tube for a color television set, by scanning a phosphor coating on the inside surface of the tube with an electron beam. The impact of the electrons on the phosphor materials cause the phosphors to luminesce and emit the light necessary to produce the image. In order to attract electrons to the phosphor screen it is necessary to place a voltage on the screen. This is accomplished by evaporating a thin layer of conductive material, such as aluminum, over the phosphor coating on the screen. The scanning electron beams which generate the image must penetrate the aluminum layer in order to cause the phosphors to luminesce. For this reason, it is important that the thickness of the aluminum coating across the screen be fairly uniform in order to prevent variations in the light output of the tube.
Typically, in the prior art, the aluminum is distributed onto the phosphor screen by placing the screen into an evacuated chamber which includes a source of aluminum. The aluminum is provided in one or more heaters which cause the aluminum to vaporize and to coat the screen. Typically, when evaporating a layer of material on a large surface, the thickness distribution of the coating material is a function of the number of evaporation sources, the location of the sources with respect to the surface being coated, vacuum conditions within the evacuated chamber, and the size of the panel being coated. In an evaporation system where the evaporation material is replenished for each evaporation, it is desirable to have the fewest number of sources while still obtaining an acceptable distribution on the surface. A single source is not practical because experiments have shown that it is not effective for large rectangular surfaces. Four sources usually suffice if the sources are properly positioned. However, this is disadvantageous because the number of sources increases the needed maintenance, the functions to be performed by the operator and the expense of the operation. Two sources can also be used. However, typically the ratio of the thickest portion of the coated film to the thinnest portion of the film is between 4:1 and 5:1. Thickness ratios of this order are unacceptable for most CRTs, and particularly for color television picture tubes, because changes of this order in the thickness of the evaporated layer cause the light output across the tube to vary between dark and light areas. For these reasons, there is a need for an apparatus for controlling the distribution of evaporated material onto a substantially flat surface, such that the ratio between the thickest and thinnest portions of the coated layer have a ratio of less than 2:1 and preferably between 1.5:1.00 and 1.00:1.00. The present invention fulfills this need.